1. Field of the Invention
The present invention relates to a method for charging a secondary battery such as a lead/lithium ion type battery and a charger to be used therefor.
2. Description of the Related Art
As shown in FIG. 1 which is a block diagram of one example of a conventional charger, the current from a power supply 101 is made constant by a constant current circuit 102 and then supplied to a lithium/ion type secondary battery 104 through a constant voltage circuit 103. Consequently, the secondary battery 104 is charged with a constant current.
Thus, when the voltage of the secondary battery 104 becomes equal to full charging voltage Vc due to the above operation, a constant voltage circuit 103 operates as shown in FIG. 2. The current I outputted from the constant current circuit 102 is gradually reduced to prevent the voltage V of the secondary battery 104 from becoming higher than the full charging voltage V.sub.c. Then, when the current I becomes smaller than a predetermined value, the charging operation is complete.
Now, the secondary battery 104 is actually composed of an ideal battery and an inner resistor r as shown in FIG. 3 (a). Further, the constant voltage circuit 103 is also composed of an ideal constant voltage circuit and an inner resistor r.sub.0 as shown in FIG. 3(b).
Accordingly, there has been the problem that while the secondary battery 104 is being charged, the voltage V of the secondary battery 104 is as shown by the solid line in FIG. 2 but when the charging operation ceases, the voltage V is reduced as shown in the dotted line in FIG. 2 due to the voltage drop across the inner resistor r or r.sub.0.
Therefore, there has been proposed a method in which the battery charging time with a constant voltage is extended to reduce the lowering of the charging voltage due to the voltage drop across the internal resistors r and r.sub.0. However, this method has had the problem of increased charging time.
Further, as described above, since the constant voltage circuit 103 has the inner resistor r.sub.0, when the constant voltage circuit 103 is in operation, a smaller the current flowing through the circuit 103, will result in a lower loss due to the inner resistor r.sub.0.
However, this method also has had the problem that since a comparatively large current flows through the constant voltage circuit 103 as shown by the solid lines marked I in FIG. 2 immediately after the constant voltage circuit 103 has started, the voltage drop is initially large.
FIG. 4 shows a second example of a conventional secondary battery charger. As shown, this charger is so constructed that a chargeable battery (secondary) 201 is charged with a constant current from a constant current circuit 202. If the battery 201 is of nickel-hydrogen type or nickel-cadmium type, the charging characteristic of the battery will be as shown in FIG. 5.
Thus, in FIG. 5, when the battery 201 is charged with a constant current I.sub.0 output from the constant current circuit 202, the terminal voltage V of the battery 201 increases gradually as the charging operation progresses and if a full charged state is achieved (i.e., a state in which the battery is charged neither too much nor too little), the terminal voltage V drops to some degree. If, therefore, the charging operation is stopped when such voltage drop (-.DELTA.V) is detected, it is possible to complete the charging operation in a state in which the battery 201 is properly charged.
However, where the battery 201 is of nickel-hydrogen type or nickel-cadmium type, the terminal voltage 201 drops when the battery is fully charged in FIG. 5, but where the battery is of lead-lithium type, the terminal voltage V of the battery does not drop when the battery is fully charged. Consequently, when the lead/lithium type battery (i.e., a lead battery, lithium battery or lithium-ion type battery, which will be hereinafter merely referred to as a battery of lead-lithium ion type), no fully charged state is detected and it is difficult to properly stop the charging operation. That is, if the charging operation is stopped when the terminal voltage of the battery reaches a value of V.sub.0, the battery is not yet in its fully charged state but in an insufficiently charged state. However, if the battery is charged for a long time after the terminal voltage of the battery reaches the value V.sub.0, the battery will become overcharged and destroyed in the worst case.
For the above reason, where a lead/lithium ion type battery is charged, the circuit structure shown in FIG. 6 is used. In FIG. 6, the lead/lithium-ion type battery 211 is charged by a constant current circuit 212 or a constant voltage circuit 213 through a switch 214. The switch 214 is kept switched upward in FIG. 6 when the battery is first charged and the battery 211 is charged with a constant current I.sub.0 outputted from the constant current circuit 212. Then the switch 214 is switched downward in FIG. 6 at a predetermined time so that the battery 211 is charged with a constant voltage V.sub.0 outputted from the constant voltage circuit 213.
In FIG. 7 which shows charging curves at the time of charging the battery 211 through the circuit shown in FIG. 6, as the charging operation with the constant current I.sub.0 progresses, the terminal voltage V of the battery 211 gradually increases and when the switch 214 is operated to allow a charging operation with the constant voltage V.sub.0 to be effected from the charging operation with the constant current I.sub.0, the terminal voltage V remains V.sub.0 while the charging current I.sub.0 decreases gradually as shown in FIG. 7. The charging current I is measured during this charging operation and when the measured value is less than a predetermined value, the battery becomes fully charged and the charging operation is stopped.
Thus, when the battery 211 is charged by the constant current circuit 212 and the constant voltage circuit 213, it is possible to charge the battery 211 in an optimum state without destroying the battery.
However, this method of switching from the constant current charging operation to the constant voltage charging operation has had the problem of requiring a long charging time.